Black Walnut Trees and Juglone

By Therese Ciesinski, GardenSMART

The woods, a meadow, your garden. To our eyes these may be tranquil scenes, plants growing in harmony with each other. And that is true, but there's also great competition for the resources that keeps each plant alive. Over the eons some plants created biological alliances with other plants, soil microbes or wildlife to survive and secure their long-term place in nature.

Others went in the opposite direction, and developed chemical defenses to keep other plants away. These chemicals inhibit the germination of nearby seeds, or weaken and kill neighboring plants. To give itself the advantage in accessing water, sun and nutrients, a plant produces its own herbicide.

The best-known – or notorious – plant with this ability is the black walnut (Juglans nigra). In this tree, the chemical that inhibits the growth of surrounding plants is juglone, and is found in various concentrations throughout the entire tree. It's most potent in the root zone, emanating into the soil as far as 50 to 60 feet beyond the trunk. Hickories (Carya spp.) and butternut (Juglans cinerea) also produce juglone.

Though this makes the black walnut sound like the thug of the forest, not every plant in its toxic orbit is affected by juglone. Also, the chemical's reach isn't infinite. Though juglone is released into the soil through the roots, it is not water-soluble, so it doesn't travel far. Neighboring plants must be within inches of black walnut roots to be affected.

Vegetables, especially those in the nightshade family, and annual flowers are most affected by juglone, though certain perennials, trees, vines and shrubs are vulnerable, too. The following is only a partial list.

Plants affected by juglone include:

Asparagus

Azalea

Bean

Birch

Blueberry

Cucumber

Eggplant

Hydrangea

Pepper

Petunia

Pine

Potato

Rhododendron

Tomato

These plants are resistant to juglone:

Begonia

Black raspberry

Clematis

Coneflower

Crabapple

Daffodil

Forsythia

Kentucky bluegrass

Liriope

Maple

Onion

Pansy

Sedum

Tulip

Symptoms of juglone poisoning include wilting leaves that look like drought stress; only the plant doesn't recover after watering. Leaves can yellow, pucker, and drop off. Plants eventually die and there's no cure once the plant is affected. It can be difficult to tell the difference between juglone toxicity and numerous other cultural problems and diseases, but if a formerly healthy plant grows into the root zone of a black walnut, it will die within a few months.

There's not much to be done to prevent damage to susceptible plants, other than to keep them away from the root zone of black walnuts, butternuts and the like. Chopping down the tree won't solve the problem, because it can take years for the juglone-filled roots to decay. Raised beds or containers for vegetables and other sensitive plants are one solution.

Keep any debris that falls from these trees cleaned up. While fallen leaves, nuts, twigs and branches contain less juglone and so are less likely to cause problems, don't compost them or use them as mulch.

Black walnut and its cousins aren't the only plants that produce chemicals that harm other plants; there are many. It is one of the most notable, however, because it is a majestic tree native to the U.S., valued for its wood, and in abundant supply.

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